1. Field of the Invention
The present invention relates to a method for creating a stepped structure on a substrate, and in particular to a self-adjusting method for creating a stepped structure, such as a gate oxide for an MOS transistor.
2. Description of the Related Art
In some applications for the production of semiconductor devices, it may be desirable to form stepped structures. One such application involves for example certain MOS transistors (MOS=metal oxide semiconductor), such as a LDMOS transistor (LDMOS=laterally diffused metal oxide semiconductor). It is advantageous if the dielectric of a gate electrode, the gate oxide, has a lower thickness on the source side than on the drain side, so that a so-called step gate oxide is formed. This step gate oxide links the demands for a lower turn-on resistance Ran, i.e. a gate oxide as thin as possible, and for a high electrical strength, i.e. a gate oxide as thick as possible.
In the prior art, methods are already known by means of which such an oxide step can be produced, and these methods are described as “dual-gox” processes in which after the production of the oxide step an adjustment of the gate electrode to this step is required.
This known method is not self-adjusting, so that restrictions result due to the limited adjusting possibilities of the exposure instruments used. The disadvantage of the gate structures produced in this way is that a particular gate length cannot be fallen short of and certain tolerances cannot be achieved.
A further disadvantage of the conventional methods is that a different doping of source regions and drain regions, as it is for example required in a LDMOS transistor, is possible only in a very difficult manner or even not at all in short gate structures.